3g lte basics tutorial

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Ericsson Review No. 2, 2008 77

Background

Mobile broadband based on high-speed

packet access (HSPA) technology is alreadya great success. But even so, to meet futuredemands for mobile broadband services, theindustry must further improve service deliv-ery, for example, through higher data rates,shorter delays, and even greater capacity.These are the very targets of 3GPP radio-access networks, specically through HSPAEvolution and LTE.1-2

Ericsson is committed to the developmentof HSPA and LTE as can be seen through anactive driving role in standardization andopen prototyping. Examples of improvedperformance compared with early 3G systems

include peak data rates in excess of 300Mbps,delay and latencies of less than 10ms, andmanifold gains in spectrum efciency. LTEcan be deployed both in new and existingfrequency bands and it facilitates simple op-eration and maintenance.1 In addition, LTEboth targets smooth evolution from legacy3GPP and 3GPP2 systems and constitutesa major step toward IMT-Advanced (Inter-national Mobile Telecommunication – Ad-vanced, sometimes referred to as 4G). In fact,LTE includes many of the features originallyconsidered for a future 4G system.

Radio interface basics

An intrinsic characteristic of radio communi-cation is that the instantaneous radio-channelquality varies in time, space, and frequency.This includes relatively rapid variations dueto multipath propagation. Radio-channelquality is thus dependent on the detailedstructure of reected radio waves (Figure 1).

Traditionally, methods for mitigatingthese variations (that is, different kinds of diversity transmission) have been employed

to maintain a constant data rate over the ra-dio link. However, for packet-data services,end-users do not usually notice rapid short-term variations in the instantaneous data rate.Consequently, one of the fundamental prin-

ciples of LTE radio access is to exploit ratherthan suppress rapid variations in channelquality in order to make more efcient use

of available radio resources. This is done inboth the time and frequency domains usingorthogonal frequency-division multiplexing-based (OFDM) radio access.

Conventional OFDM with data transmit-ted over several parallel narrowband subcar-riers lies at the core of LTE downlink radiotransmission. The use of relatively narrow-band subcarriers in combination with a cy-clic prex makes OFDM transmission inher-ently robust to time dispersion on the radiochannel, effectively eliminating the needfor complex receiver-side channel equaliza-tion. In the downlink this is a particularly

attractive property because it simplies re-ceiver baseband processing and thus reducesterminal cost and power consumption. Thisis especially important given the wide trans-mission bandwidths of LTE and – even moreso – when used in combination with multi-stream transmission.

In the uplink (where there is signicantlyless available transmission power comparedto the downlink) one of the most import-ant factors is a power-efcient transmissionscheme, in order to maximize coverage andlower terminal cost and power consump-

tion. Consequently, the LTE uplink employssingle-carrier transmission in the form of DFT-spread OFDM (also called single-carrier FDMA). This solution has a smallerpeak-to-average-power ratio than regularOFDM, resulting in more power-efcientand less complex terminals.

The basic radio resource for OFDMtransmission can be described as a two-dimensional time-frequency grid that corre-sponds to a set of OFDM symbols and sub-carriers in the time and frequency domains.

In LTE, the basic unit for data transmissionis a pair of resource blocks that correspondto a 180kHz bandwidth during a 1ms sub-frame (Figure 1). Therefore, by aggregatingfrequency resources and by adjusting trans-mission parameters, such as modulation or-der and channel code rate, one can exiblysupport a wide range of data rates.

Key features

Several key features are needed to achievethe aggressive performance targets that havebeen set for LTE. In the text that follows

we complement the basic description of sev-eral individual key features with the specictargets they address (for example, coverage,capacity, data rate, delay) – where possible,using the example depicted in Figure 1.

Spectrum exibility

Depending on regulatory aspects in differ-ent geographical areas, radio spectrum formobile communication is available in differ-ent frequency bands in different bandwidths,and comes as both paired and unpaired spec-trum. Spectrum exibility, which enables

Key features of the LTE radio interface

Erik Dahlman, Anders Furuskär, Ylva Jading, Magnus Lindström and Stefan Parkvall

This article presents some of the key features of the radio interface for LTE

(long-term evolution), recently approved by 3GPP. LTE enables unprec-

edented performance in terms of peak data rates, delay, and spectrum

efciency.

The authors discuss spectrum exibility, multi-antenna technologies,

scheduling, link adaptation, power control, and retransmission handling.

TERMS AND ABBREVIATIONS

3GPP Third Generation Partnership Project ARQ Automatic repeat request/query

Beamforming Use of multiple transmit antennas to improve signal quality at the receiverDFT Discrete Fourier transformFDD Frequency-division duplexFDMA Frequency-division multiple accessFSTD Frequency-shift time diversity

HARQ Hybrid ARQHSPA High-speed packet accessLTE Long-term evolution

Multistream Use of multiple transmit antennas to transmit several streams of datatransmission

OFDM Orthogonal frequency-divis ion multiplexingSFBC Space-frequency block codingTDD Time-division duplex VoIP Voice over IP



Ericsson Review No. 2, 200878

operation under all these conditions, is one of the key features of LTE radio access.

Besides being able to operate in differ-ent frequency bands, LTE can be deployedwith different bandwidths ranging from ap-proximately 1.25MHz (suitable for the initialmigration of, for example, cdma2000/1xEV-DO systems) up to approximately 20MHz.Furthermore, LTE can operate in both pairedand unpaired spectrum by providing a sin-gle radio-access technology that supports

frequency-division duplex (FDD) as well astime-division duplex (TDD) operation.

Where terminals are concerned, FDD canbe operated in full- and half-duplex modes.Half-duplex FDD, in which the terminalseparates transmission and reception in fre-quency and time (Figure 2), is useful becauseit allows terminals to operate with relaxedduplex-lter requirements. This, in turn,reduces the cost of terminals and makes itpossible to exploit FDD frequency bandsthat could not otherwise be used (too narrowduplex distance). Together, these solutionsmake LTE t nearly arbitrary spectrum al-

locations.One challenge when designing a spectrum-

exible radio-access technology is to preservecommonality between the spectrum and du-plexing modes. The frame structure that LTE

uses is the same for different bandwidths andsimilar for FDD and TDD.

Multi-antenna transmission

The use of multi-antenna transmission tech-niques in mobile-communication systemsenhances system performance, service capa-bilities, or both. At its highest level, LTEmulti-antenna transmission can be dividedinto

transmit diversity; and•

(pre-coder-based) multistream transmission•including beamforming as a special case.

In Figure 1, the example fading patterns fortwo users can equivalently represent the sig-nals received by a single user from two differ-ent transmit antennas. In this context, trans-mit diversity can be seen as a technique foraveraging the signals received from the twoantennas, thereby avoiding the deep fadingdips that occur per antenna.

LTE transmit diversity is based on space- frequency block coding (SFBC) techniquescomplemented with frequency-shift time di-versity (FSTD) when four transmit antennas

are used. Transmit diversity is primarilyintended for common downlink channelsthat cannot make use of channel-dependentscheduling. It can, however, also be appliedto user-data transmission – for example,

voice over IP (VoIP), where relatively lowuser data rates do not justify the additionaloverhead associated with channel-dependentscheduling. In summary, transmit diversitytechniques increase system capacity and cellrange.

Multistream transmission employs multipleantennas at the transmitter (network) and re-ceiver (terminal) side to provide simultaneoustransmission of multiple parallel data streamsover a single radio link. This technique signif-

icantly increases the peak data rates over theradio link – for example, given four base sta-tion transmit antennas and four correspond-ing receive antennas at the terminal side, onecan deliver up to four parallel data streamsover the same radio link, effectively increas-ing the data rate by a factor of four.

In lightly loaded or small cell deploy-ments, multistream transmission yieldsvery high data rates and makes more ef-cient use of radio resources. In otherscenarios – for example, large cells andheavy load – the basic channel qual-ity does not allow for extensive multistream

transmission. In this case, the multiple trans-mit antennas are best used for single streambeamforming in order to enhance the qualityof the signal.

In the context of Figure 1, beamforming

Figure 1

Channel-quality variations in frequency

and time.



Ericsson Review No. 2, 2008 79

can be seen as a means of controlling the fad-ing pattern by pre-coding the transmittedsignals to adjust the phases so that fadingpeaks appear at the receiver.

In summary, to yield good performanceover a broad range of scenarios, LTE providesan adaptive multistream transmission schemein which the number of parallel streams canbe continuously adjusted to match the in-stantaneous channel conditions.

When channel conditions are very good,•up to four streams can be transmitted in

parallel, yielding data rates up to 300Mbpsin a 20MHz bandwidth.When channel conditions are less favor-•able, fewer parallel streams are used. Themultiple antennas are instead partly usedin a beamforming transmission schemethat improves overall reception qualityand, as a consequence, system capacity andcoverage.To achieve good coverage (for instance, in•large cells or to support higher data rates atcell borders), one can employ single streambeamforming transmission as well as trans-mit diversity for common channels.

Scheduling and link adaptation

In general, scheduling refers to the processof dividing and allocating resources betweenusers who have data to transfer. In LTE, dy-namic scheduling (1ms) is applied both tothe uplink and downlink.

Scheduling should result in a balance be-tween perceived end-user quality and overallsystem performance. Channel-dependent sched-

uling is used to achieve high cell throughput.Transmissions can be carried out with higherdata rates (the result of using higher-ordermodulation, less channel coding, additionalstreams, and fewer retransmissions) by trans-mitting on time or frequency resources withrelatively good channel conditions. This way,fewer radio resources (less time) are con-sumed for any given amount of informationtransferred, resulting in improved overallsystem efciency. Figure 1 illustrates howradio channels with fast fading vary for two

users. The OFDM time-frequency grid fa-cilitates the selection of resources in the timeand frequency domains.

For services with small payloads and regularpacket arrivals, the control signaling requiredfor dynamic scheduling might be dispropor-tionately large relative to the amount of userdata transmitted. For this reason, LTE alsosupports persistent scheduling (in addition todynamic scheduling). Persistent schedulingimplies that radio resources are allocated to auser for a given set of subframes.

Link-adaptation techniques are employedto make the most of instantaneous channel

quality. In essence, link adaptation adaptsthe selection of modulation and channel-coding schemes to current channel condi-tions. This in turn determines the data rateor error probabilities of each link.

Uplink power control

Power control is about setting transmit pow-er levels, typically with the aim of

improving system capacity, coverage, and•

user quality (data rate or voice quality);andreducing power consumption.•

To reach these objectives, power-controlmechanisms typically attempt to maximizethe received power of desired signals whilelimiting interference.

The LTE uplink is orthogonal, which is tosay there is, at least in the ideal case, no inter-ference between users in the same cell. Theamount of interference to neighbor cells de-pends, among other things, on the position

of the mobile terminal – more specically,on the path gain from the terminal to thesecells. In general, the closer a terminal is to aneighboring cell the stronger the interferenceto that cell. Accordingly, terminals that arefarther away from the neighboring cell maytransmit with higher power than terminalsthat are near to the cell. In addition, there isa correlation between proximity to the serv-ing cell and distance from neighboring cells.

The LTE uplink power control takes allthese characteristics into consideration. Theorthogonal LTE uplink makes it possible tomultiplex signals from terminals with differ-

ent received uplink power in the same cell.In the short term, this means that insteadof compensating for peaks in multipath fad-ing by reducing power, one can exploit thesepeaks to increase the data rates by means of scheduling and link adaptation. In the longterm, one can set the received power targetbased on the path gain to the serving cell,giving terminals that generate little interfer-ence a larger received power target. In the

Figure 2

LTE spectrum (bandwidth and duplex)

exibility. Half-duplex FDD is seen from a

terminal perspective.



Ericsson Review No. 2, 200880

context of Figure 1, this corresponds to rais-ing the average signal level.

Retransmission handling

In practically any communications system,occasional data transfer errors arise from, forexample, noise, interference, and fading. Re-transmission schemes are used to safeguardagainst these errors and to guarantee thequality of transferred data. The more ef-cient the retransmission handling, the betteruse one can make of the radio capabilities. To

make the most of its high-performance radiointerface, LTE supports a dynamic and ef-cient two-layered retransmission scheme: Afast hybrid automatic repeat request (HARQ)protocol with low overhead feedback and re-transmissions with incremental redundancyis complemented by a highly reliable selec-tive repeat ARQ protocol.

The HARQ protocol gives the receiverredundancy information that enables it toavoid a certain amount of errors. The HARQretransmissions also provide additional re-dundancy, should the initial transmissionnot be sufcient to avoid errors. In addi-

tion, the ARQ protocol provides a means of completely retransmitting packets that theHARQ protocol could not correct.

This design yields low latency and over-head without sacricing reliability. Most er-rors are captured and corrected by the light-weight HARQ protocol; therefore, the moreexpensive (in terms of latency and overhead)

ARQ retransmissions are only rarely needed.In LTE, the HARQ and ARQ protocols

are terminated in the base station, whichgives tighter coupling between the HARQand ARQ protocol layers. The benets of this architecture are manifold, including fasthandling of residual HARQ errors and vari-able ARQ transmission size.

Conclusion

The building blocks of the LTE radio inter-

face are the key to its high performance.Spectrum exibility – in terms of variable

frequency bands, bandwidths, and FDD andTDD operation – makes LTE suitable forjust about any available spectrum.

LTE supports several features that exploitinstantaneous radio conditions in a con-structive way. Channel-dependent schedul-ing allocates the very best resources to users;multi-antenna technologies are employed tomake fading conditions on resources evenmore favorable; and link-adaptation tech-niques adapt the modulation and codingscheme to the achieved signal quality. In

the uplink, a power-control mechanism isemployed to allow very high average signalquality, and to control interference.

Aggressive use of these features is madepossible thanks to a combination of rapidretransmission of data and soft-combiningof transmission, which yields incrementalredundancy.

REFERENCES

Bergman, J., Ericson, M., Gerstenberger,1.D., Göransson, B., Peisa, J. and Wager, S.:HSPA Evolution – Boosting the performance

of mobile broadband access. EricssonReview, Vol. 85(2008):1, pp. 32-37

Beming, P., Frid, L, Hall, G., Malm, P.,2. Noren, T., Olsson, M. and Rune G.: LTE-SAEarchitecture and performance. Ericsson

Review, Vol. 84(2007):3 pp. 98-104Dahlman, E. et al: 3G Evolution: HSPA3.and LTE for Mobile Broadband. AcademicPress, Oxford, UK, 2007.

3g lte basics tutorial

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